1. Field of the invention
Apparatuses consistent with the present invention relate to a variable inductor, and more particularly, to an inductor for varying inductance according to an external control signal.
2. Description of the Related Art
In general, a communications device such as a mobile phone employs semiconductor chip elements to implement a circuit for radio frequency communications. An inductor element is essential to implement such semiconductor chip elements. In particular, a voltage control oscillator (VCO) including an inductor may be used to configure a radio frequency communications circuit for communication at a desired frequency. In these and other applications, an inductor is needed that is compact in design while also having a high inductance and quality factor.
FIG. 1 is a view showing an example of a general VCO.
Referring to FIG. 1, a VCO 10 includes an LC tank 11 consisting of an inductor L and a capacitor C and a negative resistance part 12 consisting of a pair of cross-coupled transistors M1 and M2. The VCO 10 outputs an oscillation frequency according to a resonance frequency of the LC tank 11. Accordingly, inductance of the inductor L of the LC tank 11 varies so that the oscillation frequency varies.
As wireless communications services develops, different frequency bands are used. Examples of such different frequency bands include 800 MHz for cell phones, 1.9 GHz for Personal Communications Services (PCS), and 2.4 GHz and 5 GHz for wireless Local Area Networks (LAN), and other various frequency bands. Accordingly, a multi-band VCO that is capable of providing at least two radio frequencies (RF) that are used in the different frequency bands is required and a variable inductor providing a varying inductance is also needed.
FIG. 2 is a view showing an example of a related art variable inductor and is disclosed in
U.S. Patent Publication No. 2004/0140528. Referring to FIG. 2, plural inductors 21-28 are stacked in order on a substrate and plural switches 31-33 are on/off controlled according to an external control signal so that inductance varies by the plural inductors 21-28. In the structure of such variable inductors as shown in FIG. 2, the inductors are stacked on the substrate so that additional space is not needed. However, the distance between the substrate and the inductors becomes short. As a result, Q-factor is low and the height of the structure is increased.
FIGS. 3A-3C are views showing another example of a related art variable inductor and is disclosed in U.S. Patent Publication No. 2004/0190217. Referring to FIGS. 3A-3C, the related art variable inductor includes the first inductor 42, which is fixed in position, and the second inductor 44, which is movable in left and right directions. As shown in FIG. 3A-3C, inductance varies according to the movement of the second inductor 44. However, a Micro-Electro-Mechanical Systems (MEMS) process is used so that it is difficult for such an implementation be integrated in one chip.
FIGS. 4A and 4B are views showing yet another example of a related art variable inductor and is disclosed in U.S. Patent Publication No. 2005/0068146. Referring to FIG. 4A, the related art variable inductor 50 includes the first inductor 51 which is spiral and the second inductor 52 which is loop-shaped and open and closed by a switch 53. FIG. 4A shows that when the second inductor 52 is open, the electric current does not flow in the second inductor 52 and inductance of the variable inductor 50 depends on the first inductor 51. FIG. 4B shows that when the second inductor 52 is closed by the switch 53, magnetic flux is generated in the direction of canceling magnetic flux by the electric current flowing in the first inductor 51 due to eddy current flowing in the second inductor 52. Accordingly, inductance of the variable inductor 50 in FIG. 4B becomes lower than that in FIG. 4A. In the variable inductor 50 of FIGS. 4A and 4B, the variation rate of inductance is low because inductance variation depends on eddy current flowing in the second inductor 52 according to the on or off state of the switch 53.